1. Field of the Invention
The present invention relates generally to a communication system. More particularly, the present invention relates to a system and method for probing a channel in an Orthogonal Frequency Division Multiplexing (OFDM) packet transmission system.
2. Description of the Related Art
The OFDM scheme is a multi-carrier modulation scheme that is capable of increasing transmission efficiency by obtaining Multi-User Diversity (MUD) gain using an Opportunistic Scheduling (OS) technique in a multi-user wireless transmission/reception packet-based system. The OFDM scheme is commonly used with various digital transmission technologies.
The OFDM scheme, though it is similar to the conventional Frequency Division Multiplexing (FDM) scheme, has a special characteristic in that it can obtain optimal transmission efficiency during high-speed data transmission by maintaining the orthogonality between multiple subcarriers during their transmission. The OFDM scheme can obtain optimal transmission efficiency during high-speed data transmission because it has high frequency utilization efficiency and is robust against multi-path fading. Further, the OFDM scheme, as it overlaps frequency spectrums, not only has high frequency utilization efficiency and is robust against frequency-selective fading and multi-path fading, but it can also reduce an inter-symbol interference effect using a guard interval, can facilitate the design of a hardware structure of an equalizer and is robust against impulse noises. For these reasons, it is commonly used for a communication system configuration.
The multi-user wireless transmission/reception packet-based Time Division Duplex (TDD) system determines a Modulation and Coding Scheme (MCS) and selects a transmission band according to a Channel Quality Indicator (CQI) of the downlink when transmitting uplink data. However, the system performance may decrease due to a CQI feedback delay problem and/or an information quantization error. In the Frequency Division Duplex (FDD) system, unlike in the TDD system, the uplink/downlink channel characteristics are different from each other. Therefore, in the FDD system it is essential to send a probing signal in order to acquire frequency channel information.
The channel probing used in the conventional Institute of Electrical and Electronics Engineers (IEEE) 802.16e system allocates consecutive probing subchannels in the last region of the uplink and employs them. According to the prior art, a base station (BS) provides each user (e.g. mobile station (MS)) with the information indicating the start position and the number of probing transmission subchannels using a downlink channel. Because the probing transmission information delivered from the base station is composed of only the start position and the number of subchannels, each user transmits as many probing signals as the number of subchannels consecutively allocated, beginning at the start position.
However, because the above-described method consecutively allocates the probing transmission subchannels, it needs to probe signal transmissions over the full frequency band to fully detect the channel state of the full band. Accordingly, its probing burden is very high. In addition, because this method can acquire the channel characteristic only for the subchannels undergoing the probing, the decrease in the number of allocated probing transmission subchannels decreases the possibility that the optimal transmission region will be selected for each user. This causes a decrease in the total system performance.
Further, the consecutive allocation of the probing transmission subchannels cannot provide the frequency diversity effect for selecting the optimal transmission region. That is, because the inter-channel correlation is high between adjacent frequency bands, the probing performed using the consecutive subchannels decreases the channel variation between the transmission probing frequency regions, causing a decrease in the probing effect.
In an OFDM system, to apply the OS method, the base station needs full-band instantaneous Signal-to-Noise Ratios (SNRs) for all users. However, this causes a considerable probing signaling burden. In addition, an increase in the moving velocity of the user requires a greater amount of probing information to prevent the performance reduction. When the total number of users of the system is denoted by K, the number of subchannels is denoted by M, a length of the time domain is denoted by T, and the number of bits of a probing signal needed to obtain an instantaneous SNR is denoted by B, the entire overhead of the probing signal required for OS is KMB/T bps. Therefore, an increase in the total number of subchannels or users of the system causes a linear increase in the burden of the probing signals.
Accordingly, there is a need for a transmission scheme for maximizing the system performance while minimizing the probing burden.